1. Technical Field
The present invention relates to a display apparatus for displaying a three-dimensional image, particularly to a method of disposing elemental images to secure a maximum viewing zone, a method in which an observer can recognize deviation from the viewing zone, and an apparatus for realizing this method.
2. Description of the Related Art
There are various systems for displaying a three-dimensional image, and there may be roughly two methods. One of them is a system for a stereoscopic viewing using a binocular parallax, and the other method is a space image reproduction system for actually forming the space image in space.
As the binocular parallax system, there have been proposed various systems with or without the presence of glasses, for example, a stereoscopic system including video information for left/right eyes (so-called stereoscopic method) and a multi-view system in which a plurality of observation positions at the time of video photography are disposed to increase an amount of information and to expand an observable range. Here, in the stereoscopic system, two photography positions are disposed for left and right eyes to obtain images for left and right eyes so that the images are visible with the left and right eyes, respectively. In the multi-view system, the video photography positions are further increased as compared with the stereoscopic system.
In contrast to the binocular parallax system, the space image reproduction system in which the image is reproduced in the space is an idealistic system for three-dimensional image reproduction, and holography is classified in the space image reproduction system. This system is sometimes referred to as an integral imaging (II) system proposed by Lippmann in France in 1908 (also referred to as an integral photography (IP) system, and sometimes as a light ray reproduction method), and is also sometimes classified in the binocular parallax system. However, in the II system, light rays travel in reproduction optical paths at the time of reproducing, which are opposite to photographing optical paths at the time of recording to reproduce a complete three-dimensional image. Therefore, an ideal integral imaging (II) system must be classified in the space image reproduction system.
This integral imaging (II) system or the light ray reproduction system has been disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication Nos. 10-239785 and 2001-56450. Here, meanings of terms of the integral imaging method and light ray reproduction method are not exactly established in a stereoscopic display method, but the methods may be considered based on about the same principle. In the following description, the system will be referred to as the “integral imaging system” as a concept including the light ray reproduction system.
In recent years, a system which includes a lenticular lens or a parallax slit combined with a display has come into the mainstream for displaying three-dimensional images. In such a system, stereoscopic observation is possible without any glasses. If the three-dimensional image is displayed without any glasses as in the multi-view system or the II system, the following system or apparatus is sometimes used. In the system, a display unit is provided, which has a plurality of two-dimensionally arranged pixels, and an optical barrier is disposed in front of the pixels. The pixels display a two-dimensional image, which is projected in a front space through the optical barrier to form a three-dimensional image. The optical barrier has openings, slits, or exit pupils, each of which has a size smaller than that of the pixels, typically, so as to pick up one image from the pixel. The optical barrier has a function of controlling a transmission of light rays emitted from the pixels so that the optical barrier is called a transmission control unit and the opening is called a transmission control section, and the openings are realized by pinholes or micro lenses, which are two-dimensionally arranged. In a three-dimensional image display apparatus, a natural three-dimensional image can be reproduced. A liquid crystal display unit can be used as the display unit, which includes image display elements corresponding to the pixels arranged in a matrix form.
A large number of elemental images each composed of two-dimensional images are displayed on the pixels for the three-dimensional image, which are observed subtly different in a visible way in accordance with a viewing angle, wherein each of the pixels have a positional relationship with the individual pinholes or micro lenses on the three-dimensional image display apparatus. That is, light rays are emitted to the front space of the display apparatus from the elemental images through the corresponding pinholes or micro lenses, or from a light source through the pinholes or micro lenses and the elemental images. These light rays form a three-dimensional real image in front of the transmission control sections, such as the pinholes or the micro lenses. When paths of these light rays are extrapolated on the rear space in the back side of the transmission control sections of the pinholes or the micro lenses, a three-dimensional virtual image (image which does not exist as viewed on a rear-space side) is observed on the rear space of the transmission control sections of the pinholes or the micro lenses. That is, as observed by the observer, the three-dimensional real image is observed by a group of light rays which are emitted from the elemental image and form the image in the front space of the transmission control sections, and the three-dimensional virtual image is observed by the group of light rays which also form the image in the rear space of the transmission control sections.
As described above, various systems for displaying the three-dimensional image in real space have been proposed. In an ultimate three-dimensional image display, the displayed image seems to be natural as if the displayed image actually existed in the real space. From this standpoint, the integral imaging system in which convergent points are not located at a viewing distance is assumed to be a superior method, because a natural stereoscopic image can be formed by a simple construction. Since the visible images are successively changed in accordance with the angle viewed by the observer through the windows, a natural motion parallax is obtained, and a more realistic stereoscopic image can be reproduced. In this respect, this method is superior.
It is to be noted that a multi-view display apparatus without any glasses seemingly has a construction similar to that of the display apparatus of the II system. However, the display apparatus of the multi-view system is obviously different from the II system in that the light rays from the apparatus are converged on an observation plane positioned in a visual distance. For the display apparatus of the multi-view system, the observer is requested to be positioned in or around the visual distance, and a converging point of the light rays in this visual distance is requested to be set to be (1/integer) times of an interval between the eyes. In other words, when the observer is positioned in the visual distance, the light rays passing through (1/integer) of an exit pupil need to be incident upon observer's both eyes. For the display apparatus of the multi-view system, even when the number of two-dimensional images obtained from different directions and corresponding to one exit pupil is small, the three-dimensional image can be recognized by the binocular parallax. Therefore, when the number of pixels is limited because of various circumstances such as a resolution of an image display unit in the display apparatus of the multi-view system, there is a merit that the three-dimensional image of high precision can be represented as compared with the II system. However, when the observer moves in a transverse direction in the display apparatus of the multi-view system, and when an interval between the converging points is not sufficiently short, there is a problem that flipping of the three-dimensional image is observed or that the viewing zone along the viewing direction is limited.
In the apparatus without any glasses, there is a common problem in which a viewing region may be limited, even if the three-dimensional image can be observed without glasses.
The light rays from the light source are radiated in all directions via each pixel of a transmission type display unit in which the parallax information is displayed, and transmitted through the transmission control section such as a pinhole array, a slit plate, a fly-eye lens array, or a lenticular lens array having an array of exit pupils, as described above. Accordingly, the light rays are so controlled as to bear parallax information and are projected in a predetermined direction. The light rays emitted in the predetermined direction are incident upon the observer's eyes and visually recognized by the observer's eyes in accordance with observer's eye positions, and the three-dimensional image is recognized by the binocular parallax. If a sufficiently large number of light rays are projected in the front space, the three-dimensional real or virtual image is formed in front of or behind the display unit, and the observer can recognize the image. In this specification, a combination of two-dimensional images obtained from various directions on one exit pupil is referred to as elemental images.
In the above-described display apparatus, the light rays emitted from the pixels are designed to pass through the corresponding exit pupil and are directed to a predetermined direction, but some of the light rays are actually passed through another exit pupil, especially through the adjacent exit pupil, and are directed to another direction. The light rays passing through this wrong exit pupil form an image (hereinafter referred to as a quasi image) different from the original three-dimensional image (hereinafter referred to as the correct image). The quasi image resembles the correct image, but is formed into a distorted image in accordance with a deviation of a designed value. When the wrong light rays hinder the correct image, the correct and quasi images are visually recognized in an intermingled manner.
Essentially, when the integral imaging system (II system) that is of a photographic concept is to be realized with electronic devices such as LCD and PDP, there is a concept of viewing zone, but a method of preparing the image displayed on the image display unit in consideration of the viewing zone has not been studied. If the concept of viewing zone is not introduced in the formation of the three-dimensional image to be displayed by the image display unit, there is a problem that an actual viewing zone is narrowed as described hereinafter.
In the three-dimensional image display apparatus of the II system in which the viewing zone is not considered, a positional relation between the elemental images corresponding to the exit pupil is not studied in detail. In the II system, there are provided a main viewing zone or a main lobe in which the correct image is produced, a transitional zone in which the quasi image is produced, and an intermingled zone or a side-lobe zone between the main viewing zone and the transitional zone, in which the quasi and correct images are intermingled. In any case, there is a problem that the main viewing zone only for the correct image is narrow even as compared with the intermingled transitional zones, and is a limited region, which cannot substantially be used practically.
As described above, in the three-dimensional image display apparatus in which the II system is used, a problem that the viewing zone is narrow and the region including the intermingled and visually recognized correct and quasi images is large has been pointed out.
Moreover, as described above, in the three-dimensional image display apparatus without any glasses, the observer may be shifted into the side-lobe zone from the main viewing zone, when the observer moves along the display panel. Therefore, the quasi image different from the true image may be gradually intermingled, and the quasi image may be perceived. This is because a part of the elemental image displayed by the pixel for displaying the three-dimensional image disposed adjacent to the original pixel for displaying the original three-dimensional image is visually recognized via the opening disposed opposite to the certain pixel for displaying the three-dimensional image in a case where the opening is observed from a wide field of view including the deviating observation position.
The three-dimensional image display apparatus using the above-described construction can be used in various fields, and a medical application is one of the applications of the apparatus. When the three-dimensional image display apparatus is used in the medical field, what is remarkably important is that the quasi image cannot be perceived or that the observer can recognize that the three-dimensional image perceived by the observer includes the quasi image. However, in a conventional three-dimensional image display apparatus, the quasi image is inevitably perceived. Moreover, when the perceived three-dimensional image includes the quasi image, it cannot constantly be recognized that the quasi image is observed in a case where the perceived three-dimensional image includes the quasi image.
To solve the problem, the use of refraction of light has been proposed as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-72136. It is to be noted that in this publication a color filter having a function of an optical shutter constitutes the pixel for displaying the three-dimensional image, and a white point light source array is disposed on the rear surface of the color filter instead of using the optical barrier.
In the technique described in the above-identified Jpn. Pat. Appln. KOKAI Publication No. 2002-72136, a transparent medium whose refractive index is larger than 1 is inserted between the color filter and the white point light source array. In this structure, light ray components on the side of the wide field of view can totally be reflected by the surface of the transparent medium on the side of the color filter among the light rays from each white point light source. Therefore, when a distance between the transparent medium and the color filter is appropriately set, the light rays from the white point light source disposed opposite to the pixel for displaying the three-dimensional image can be prevented from entering the adjacent pixel for displaying the three-dimensional image. Therefore, it is possible to prevent the quasi image from being perceived.
However, since the refraction of light rays is used in this method, the region of the observation position in which the true image can be perceived is broadened. Therefore, there is a problem with the image, in situations in which the observation position is moved irregularly or when the number of pixels for displaying the two-dimensional image included in the pixels for displaying the three-dimensional parallax is changed. Specifically, a natural movement pattern is lost.